The red jumper, famously worn by Diana at a polo match in June 1981 shortly after her engagement, far exceeded its estimated value of $50,000 to $80,000 (£40,000 to £70,000).

Discovered in an attic in March by designer Joanna Osborne, the sweater features a whimsical black sheep among white ones and is among the initial designs by Osborne and Sally Muir for Warm and Wonderful knitwear, which was founded in 1979.

The winning bidder's identity remains undisclosed.

Sotheby's described the sweater as featuring a "whimsical black and white sheep motif" and said it was among the pioneering creations by the knitwear brand.

The Sotheby's catalogue note says: "Worn by Diana Spencer in 1981 to a polo match of Prince Charles.

"This lot includes two official letters, written in 1981, from Buckingham Palace to Warm and Wonderful politely explaining the jumper had been damaged and requested either a repair or replacement.

"Warm and Wonderful sent her a replacement sweater that she wore in 1983 to another polo match."

Sotheby's added: "Further investigation convinced the team: the damaged cuff, the shape, the single sheep at the shoulders, tiny variations in the sheep's eyes which were individually sewn on..."

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It said that in conjunction with a photo-matched authentication by Sotheby's the lost sheep jumper found in an attic is "confirmed to be the original sheep jumper worn by Princess Diana over 40 years ago".

Online bidding began on 31 August, coinciding with the 26th anniversary of Diana's tragic death in a car accident in Paris.

The jumper gained such fame as a result of its use by Diana that a copy is held at the Victoria and Albert Museum.

Some commentators have claimed that the princess's later wearing of the garment had hidden meaning, with the single black sheep emblematic of how she viewed her place in the Royal Family - an idea hinted at in The Crown, when it is worn by Emma Corrin in a season 4 episode of the Netflix series.